Display devices for use in TVs, cell phones, etc., and optical elements, such as camera lenses, etc., usually adopt an Antireflection technique in order to reduce the surface reflection and increase the amount f light transmitted therethrough. This is because, when light is transmitted through the interface between media of different refractive indices, e.g., when light is incident on the interface between air and glass, the amount of transmitted light decreases due to, for example, Fresnel reflection, thus deteriorating the visibility.
An antireflection technique which has been receiving attention in recent years is forming over a substrate surface a microscopic uneven pattern in which the interval of recessed portions or raised portions is not more than the wavelength of visible light (λ=380 nm to 780 nm). See Patent Documents 1 to 3. The two-dimensional size of a raised portion of an uneven pattern which performs an antireflection function is not less than 10 nm and less than 500 nm. Here, the “two-dimensional size” of the raised portions refers to the area equivalent circle diameter of the raised portions viewed in a direction normal to the surface. For example, when the raised portions have a conical shape, the two size of the raised portions is equivalent to the diameter of the base of the cone. The same applies to the “two-dimensional size” of the recessed portions.
This method utilizes the principles of a so-called moth-eye structure. The refractive index for light that is incident on the substrate is continuously changed along the depth direction of the recessed portions or raised portions, from the refractive index of a medium on which the light is incident to the refractive index of the substrate, whereby reflection of a wavelength band that is subject to antireflection is prevented.
The moth-eye structure is advantageous in that it is capable of performing an antireflection function with small incident angle dependence over a wide wavelength band, as well as that it is applicable to a number of materials, and that an uneven pattern can be directly formed in a substrate. As such, a high-performance antireflection film (or antireflection surface) can be provided at a low cost.
The present applicant has developed a method of producing an antireflection film (or antireflection surface) which has a moth-eye structure with the use of an anodized porous alumina layer obtained by anodization of aluminum (Patent Documents 2 and 3).
Utilizing an anodized porous aluminum film can facilitate the manufacture of a mold which is used for formation of a moth-eye structure over a surface (hereinafter, “moth-eye mold”). In particular, as described in Patent Documents 2 and 3, when the surface of the anodized aluminum film as formed is used as a mold without any modification, a large effect of reducing the manufacturing cost is achieved. The structure of the surface of a moth-eye mold which is capable of forming a moth-eye structure is herein referred to as “inverted moth-eye structure”.
As described in Patent Documents 1 to 4, by providing an uneven structure (macro structure) which is greater than a moth-eye structure (micro structure) in addition to the moth-eye structure, the antireflection film (antireflection surface) can be provided with an antiglare function. The two-dimensional size of a raised portion or a recessed portion of the uneven structure which is capable of performing the antiglare function (sometimes referred to as “antiglare structure”) is not less than 200 nm and less than 100 μm. The structure of the surface of a mold which is capable of forming the antiglare structure is referred to as “inverted antiglare structure”. The entire disclosures of Patent Documents 1 to 4 are incorporated by reference in this specification.
In this specification, an uneven structure which forms the moth-eye structure (or inverted moth-eye structure) is referred to as “micro uneven structure”, and an uneven structure which forms the antiglare structure (or inverted antiglare structure) is referred to as “macro uneven structure”. The extent of the two-dimensional size of the macro uneven structure partially overlaps the extent of the two-dimensional size of the micro uneven structure antireflection film (artireflection surface) which has an antiglare function, the uneven structure that forms the antiglare structure is greater than the uneven structure that forms the moth-eye structure that performs an antireflection function.